A 3-level inverter produces a 3-level voltage, e.g., a high voltage, a medium voltage, and a lower voltage, by subdividing a DC power voltage (stringing voltage) into two DC voltages by way of series-connected capacitors, and selectively conducts these three leveled voltages to an inverter output terminal by turning ON/OFF the switching elements of the major circuit. The 3-level inverter has the following features.
That is, since the step number of the output voltage pulse is increased, the virtual switching frequency is increased and then an output with less distortion is obtained. Since the voltage applied to the element is reduced to approximately 1/2, as compared with that of the 2-level inverter, the switching elements with relatively low-withstanding voltages may be utilized. Since the voltages applied to the switching elements are lowered, loss occurring in the switching elements may be reduced.
As the generating/controlling method for the output voltage pulse of the above-described 3-level inverter, the following methods have been proposed:
(1) "NEW DEVELOPMENTS OF 3-LEVEL PWM STRATEGIES" (EPE' 89 Record, 1989), page 412. In FIG. 1, there are shown a so-called "dipolar modulation" (the output voltage is produced by alternately outputting the positive/negative pulses via the zero voltage within a half period of the output voltage), a so-termed "unipolar modulation" (the output voltage is produced by outputting the pulse with a single polarity within a half period of the output voltage), and also a method for switching the above-described dipolar modulation and unipolar modulation. PA1 (2) "PWM Systems in Power Converters: An Extension of the "Subharmonic" Method" (IEEE Transaction on Industrial Electronics and Control Instrumentation, vol. IECI-28, No. 4, November 1981), page 316. In FIG. 2(b), such a modulation method (will be referred to "overmodulation") has been proposed. That is, a half period of the output voltage is constructed of a plurality of pulses each having a single polarity, and the output voltage is produced by reducing the number of pulses in such a manner that slits among the pulses are filled up from the center portion of this half period. PA1 (3) STUDY OF 2 AND 3-LEVEL PRECALCULATED MODULATIONS (EPE' 91 Record, 1991), page 411. In FIG. 16, the output voltage pulse generating/controlling method has been proposed for covering the output voltage from 0 to 100%.